Because of the relatively light weight of aluminum when compared to brass or steel or similar materials, there is an ever increasing desire to utilize this advantage in aluminum cartridge cases provided such could be made to satisfy the severe requirements inherent in this application. The light weight is important especially in view of present day technology which allows for very rapid fire weapons which in the case of aircraft, for example, have the capability of firing in the vicinity of 6000 rounds of ordnance per minute. Because of this rapid fire, it is more advantageous for the aircraft to carry more ammunition; however, obviously, there are certain weight limitations that must be adhered to for the aircraft and thus all components are normally made as light as possible in order to carry larger payloads. For example, in considering a 30 mm cartridge case, an aluminum case is only about 1/3 the weight of a steel or brass case thus providing a large savings in payload. Or in the loaded cartridge, a complete aluminum cased round only weighs about 70% that of a steel or brass cased round. To further illustrate the importance in weight, a 30 mm aluminum cartridge case weighs about 1/3 pound and firing 6000 rounds of such would use about 1 ton of aluminum, the equivalent is steel or brass being about 3 tons. Thus, it can be seen that using aluminum cartridge cases results in large weight savings and the ability to carry many more rounds of ammunition.
When an aluminum alloy is used for cartridge cases, it is important that the alloy have high strength and toughness and tear resistance to ensure against jamming or tearing during the firing operation. Some aluminum alloys have the requiste strength, for instance, 7075 type, but can lack the needed tear resistance, whereas others have the necessary tear resistance, for instance, 2024 type, but can lack the yield strength. The absence of one or the other of these characteristics can result in failures or other shortcomings such as weight increase. Jamming failures often develop as a result of rim shear encountered because of the high stresses experienced during loading, firing and extracting. Here the rim on the head of the case which forms one side of an extractor groove shears and can interrupt or disturb and ejection operation. Rim failure appears to be characteristic of alloys having a relatively low yield strength. Failures by tearing can seriously interfere with the firing operation by allowing hot propellant gases to escape into the breech of the gun, which gases can melt the aluminum case or do damage to the steel parts of the gun. Aluminum alloy 7475 combines both high strength and tear resistance in sheet or plate material as indicated in U.S. Pat. No. 3,791,880 which issued in the name of Hunsicker and Staley in February 1974. This sheet or plate can be used in producing cartridge cases of high strength and toughness. However, because of the scrap losses generated by blanking discs from sheet or plate and also, the fabricating steps required to provide the sheet or plate, it can be more advantageous to form the cartridge cases from rod or bar stock. However, fabricating cartridge cases from slugs cut from a rod or bar stock formed from 7475 type alloy stock by conventional practices can result in undersirable circumferential surface cracks or checks and in certain instances after extended periods of storage in susceptibility to stress corrosion cracking in the region where the neck of the cartridge case is squeezed to grip the projectile. Such surface cracking or stress corrosion cracking can lead to failure in firing. It is readily appreciated that failure in firing is usually considered totally unacceptable in weapon systems and hence these problems must be overcome on a highly consistent and repeatabale basis.
This invention overcomes the problems encountered in fabricating an aluminum cartridge case from rod or bar stock by providing a controlled sequence of steps resulting in an improved cartridge case which is crack-free and not subject to stress corrosion cracking and which has high strength and toughness or tear resistance.